1. Field of the Invention
The present invention relates to a current position calculating system, which is installed on a mobile object such as a vehicle or the like, for measuring a travelled distance, a headed direction etc., and for calculating a current position of the vehicle according to the measured values. More particularly, it relates to techniques for correcting an error in measuring the travelled distance.
2. Description of Related Art
A current position of a vehicle is usually calculated based on a headed direction of the vehicle measured by a direction sensor such as a gyrocompass and a travelled distance of the vehicle measured by a vehicle speed sensor or a distance sensor.
Generally, a travelled distance of a vehicle is obtained by measuring the number of revolutions of an output shaft of a transmission or a tire and multiplying the result by a distance factor, which corresponds to a distance the vehicle travelled for each revolution of the tire.
In addition, there is known a technique in which in order to correct an error in the current position obtained from the headed direction and the travelled distance as described above the obtained current position of a vehicle is corrected so as to match the same with road data, what is called a map matching technique as disclosed in the official gazette of Japanese Patent Application Publication (KOKOKU) No. 13972/1994. This map matching technique can enhance an accuracy in calculating a current position.
Meanwhile, the diameter of a tire, which relates to a distance factor, varies continuously while driving, due to such factors as wearing away or expansion caused by a temperature change, of the tire. As a result, an error is generated in calculating the travelled distance and a highly accurate calculation of the current position cannot be performed. For example, one percent of error in the travelled distance factor for each revolution of the tire will result in an error of 1 kilometer if the travelled distance is 100 kilometers.
Such an error in measuring the travelled distance as described above can be corrected to a certain extent by using the aforementioned map matching technique if it is applied to driving on an ordinary road. Even though the current position can be corrected by the map matching technique at a point of intersection or the like, the technique itself cannot correct the distance factor and it is impossible to avoid an accumulation of errors in the travelled distance.
Moreover, once an error of approximately 1 kilometer is produced between the estimated current position and the actual current position, it will be difficult to correct the position according to the map matching technique.
So far, the following techniques have been employed for avoiding errors in measuring a travelled distance:
(1) correcting a distance factor by comparing the road data from the point of turning at an intersection (a starting point) to the point of turning at the next intersection (a terminal point), with the travelled distance obtained by measuring the number of revolutions of an output shift of a transmission or a tire; (2) correcting the aforementioned distance factor by comparing the distance between two beacons on a map, with the distance measured by driving as disclosed in the official gazette of Japanese Patent Application Publication (KOKOKU) No. 27652/1994; and (3) correcting the aforementioned distance factor by obtaining a vehicle speed from the GPS (Global Positioning System) receiving device for calculating a current position using the signals from the GPS satellites and comparing the obtained vehicle speed with a detected number of revolutions of a tire, as disclosed in the official gazette of Japanese Patent Application Laid-open (KOKAI) No. 107958/1990.
However, an accurate correction may not be performed in the technique (1) described above due to even a slight curve of the road or meandering of the vehicle. It is also difficult to precisely identify the aforementioned starting point and/or the terminal point.
Similarly, accurate correction may not be performed in the technique (2) in a condition where the road is not straight and there is also a problem of installing beacon equipment which can be used on a vehicle.
Moreover, in the technique (3), it may occur that accurate speed information cannot be obtained if a vehicle runs at a low speed, and an error may be produced in a calculated speed due to a long processing time in the case of a significant change in the vehicle speed, and thus there is a problem that an accurate correction cannot be performed. Furthermore, it is necessary to install GPS receiving equipment and a GPS receiving antenna which can be used on a vehicle. There is also a problem that correction is disabled because of the absence of the GPS signals received when the vehicle is in such situations as passing through a tunnel, running under an elevated track or in the shade of buildings.